The objective of the proposed study is to isolate, characterize, and genetically analyze a set of temperature sensitive (ts) mutants of the murine sarcoma virus. Particular emphasis will be placed on mutants defective in genes with respect to the expression of transformation at the non-permissive temperature. We hope to determine the number of genes involved and their role in carrying out the transformation process in the infected cells. Furthermore, we propose to investigate the relatedness and interaction of the genomes of MuLV and MuSV by means of complementation and recombination tests using ts mutants of MuLV as probes.